The differentiation capacity of isolated subsets from normal mouse thymus has been examined in vivo and in vitro. In experimental cell transfer studies using congenic mice we have determined that the most immature intrathymic subset of adult mouse thymocytes is a dull Lyl+ (dLyl) subsest which is lacking both Lyt2 and L3T4 cell surface expression. This subset is seen early in thymus graft analyses. The dLyl cells isolated from adult thymus have been shown to be capable of thymus repopulation in irradiated animals, but these cells are not capable of repopulating other hematopoietic compartments. Thus the dLyl cells are committed thymocyte progenitors with limited capacity for repopulation. Because some of the dLyl cells can also be shown to differentiate into Lyt2+, L3T4+ cells in short term in vitro culture, we have examined the freshly isolated dLyl cells for expression of Lyt2 mRNA. Lyt2 mRNA was not detected in the freshly isolated dLyl cells, although appropriate transcripts could be seen in total thymocyte populations and selected Lyt2+ populations. A minor population of immature dLyl cells can be maintained in vitro for several months without further cell surface differentiation. We are currently examining the growth requirements and other characteristics of these cells using interleukin and mitogen stimulation and addressing questions of expression and induction of cell surface receptors for interleukins known to affect thymocyte proliferation.